Field of the Invention
The invention relates to an apparatus for connecting a converter to an AC grid, which apparatus has a grid connection for connecting to the AC grid and a converter connection for connecting to the converter, wherein the grid connection and the converter connection can be connected to one another via at least two transformers, which are connected in parallel with one another and are equipped with a tertiary winding in addition to a primary winding and a secondary winding, at least one switch being arranged in series with each transformer, and a starter unit being provided, which is connected to a tertiary winding of one of the transformers.
The invention also relates to a high-voltage system and a method for starting a non-magnetized or already pre-magnetized second transformer.
Such an apparatus and such a high-voltage system and such a method are already known in practice in the field of high-voltage DC transmission. FIG. 1 shows schematically a typical known high-voltage DC (HVDC) transmission system 1. The HVDC transmission system 1 shown is used for the transmission of electrical power between a first AC grid 2 and a second AC grid 3. In this system, each AC grid 2, 3 is connected via an apparatus 4 of the type in question to an AC terminal (not shown in the figure) of a converter 5. For power transmission, AC voltage is converted into DC voltage by a converter 5, and transmitted to the other converter 5 via a DC grid 6. The apparatus 4 is used to connect each converter 5 to its associated AC grid 2 and 3 respectively, and comprises two transformers 7, 8 connected in parallel with each other. Each transformer 7, 8 is equipped with a primary winding 9 a secondary winding 10 and a tertiary winding 11. The primary winding 9 is galvanically connected to a grid connection (not shown in the figure), and the secondary winding 10 to a converter connection (also not shown in the figure). The tertiary winding 11 is used for drawing electrical power for internal consumption by the HVDC transmission system 1, so for instance for providing light, heating power, the power supply for control equipment and the like. In principle, one transformer 7, 8 would be sufficient for power transmission between converter 5 and the AC grid. For reliability reasons, however, a second transformer 7, 8 arranged in parallel is provided in order to be able to cover for a failure of the first transformer 7. In normal operation, the energy flows via both parallel-connected transformers 7, 8. When the HVDC transmission system 1 is started up, only the first transformer 7 is connected to the converter 5. This first transformer 7 can therefore be soft-started by slowly increasing the voltages on the AC side of the converter 5 during startup of the HVDC transmission system 1. Particularly when using the HVDC transmission system 1 to connect offshore wind farms, first the primary winding 9 is connected to the AC grid 2, and the secondary winding 10 to the converter 5. Then the voltage produced on the AC side by the converter 5 is increased steadily, with the transformer 7 being slowly magnetized and simultaneously synchronized with the AC voltage of the AC grid 2. The second parallel transformer 8 can likewise already be connected to the converter during this startup process. Also a startup process using just the first transformer 7 is feasible. This is the case, for instance, when maintenance work is being performed on the parallel transformer 8. In FIG. 1, switches (not shown in the figure) are used to isolate the transformers 7, 8.
Starting the second transformer 8 while operation is in progress can interfere with the connected AC grid 2, because in some cases the short-circuit power capacities of these grids are low. This is particularly the case for the already-mentioned connection of offshore windfarms, because the windfarms together with the converter provide only a weak AC grid 2.
When starting the second transformer 8, the currents that flow can be so high that the voltage in a weak AC grid 2 collapses and drops below a permitted total voltage limit. Furthermore, distortions may arise in the AC grid 2 in the form of harmonic components of the fundamental frequency. In addition, overload or damage to the converter from excessive currents cannot be ruled out and is even probable for a weak AC grid such as an island grid, for instance. Even standard solutions in energy technology such as, for example, starting the primary or secondary winding under phase-selective control cannot always ensure that the described interference does not occur in the grid (grid distortions).
In particular, the second transformer must be started when it has previously been switched off for maintenance purposes. For commercial operation, it must be possible to maintain the installation without interrupting the transmission of energy from the system. This is another reason why the system has at least two parallel transformers. The electrical power can then be transmitted via the transformer that is connected in parallel with the switched-off transformer.
It is known in practice to connect the second transformer 8 via a tertiary winding 11 to a diesel generator that can generate a variable voltage level, before starting said second transformer. Hence the diesel generator can be used to magnetize the second transformer 8. Synchronization means are used to synchronize with the AC voltage in the AC grid 2. After being magnetized and synchronized with the AC grid, the second transformer 8 can be soft-started. The known apparatus and known method have the associated disadvantage that the synchronization requires additional components. Furthermore, fuel must be provided for the diesel generator. Particularly on offshore platforms, however, there is a limited supply of fuel. Also refueling such a platform is not trivial. Dispensing with diesel equipment is currently being discussed for future platforms, in particular those in the North Sea.